The present inventors have researched and proposed a radio optical fusion communication system with an integration of optical fiber transmission and a radio communication. Particularly, in a method proposed by the present inventors, optical signals are generated using a first laser light source and a second laser light source of different wavelengths, the first optical signal is modulated into an unsuppressed-carrier single-sideband (SSB) or double-sideband (DSB) modulated optical signal using an intermediate-frequency signal, the modulated signal is mixed with the second optical signal, and the resultant optical signal is transmitted.
The optical signal is opto-electrically converted to generate an unmodulated carrier and a modulated radio signal. In a radio propagation path, upon receiving, multiplication components of the unmodulated carrier and the modulated radio signal are obtained to extract an intermediate-frequency converted signal and the signal is then demodulated.
In the above-described self-heterodyne transmission method, a received signal can be stably reproduced. In addition, signals can be transmitted from a base station to an antenna station over a several-km optical fiber transmission path with low loss. Accordingly, a preferred communication system can be realized.
FIG. 10 is a diagram of the structure of a radio optical fusion communication system based on the above-described method. As shown in the diagram, the system includes a base station (100), a remote antenna station (110), and a receiving terminal (120). The base station (100) is connected to the remote antenna station (110) via an optical fiber transmission path (130). The remote antenna station (110) is connected to the receiving terminal (120) via a radio propagation path (131).
The base station (100) includes a first laser light source (101) for single-mode oscillation at an oscillation frequency f1 (Hz), a second laser light source (102) for single-mode oscillation at an oscillation frequency f2 (Hz), and an intermediate-frequency signal generator (103) for generating an intermediate-frequency modulating signal, which is modulated based on information signal data to be transmitted.
An intermediate-frequency signal having an intermediate frequency f_m (Hz) generated from the intermediate-frequency signal generator (103) is supplied as a modulating signal to an optical modulator (104) in the base station (100). The optical modulator (104) modulates a first optical signal from the first laser light source (101) into signal light. In this structure, a suppressed-carrier optical single-sideband (optical SSB) modulator is used as the optical modulator (104). Accordingly, an image suppressed signal with carrier is obtained.
A second optical signal from the second laser light source (102) is supplied to an optical mixer (105) without being modulated. The optical mixer (105) mixes the second optical signal with the optical signal supplied from the optical modulator (104). An optical spectrum (140) in the optical fiber transmission path (130) is shown in the figure. In other words, the spectrum includes a second optical signal (141) having the frequency f2 (Hz), a first optical signal (142) having the frequency f1 (Hz), and a modulating signal (143) having a frequency f1+f_m (Hz).
In the remote antenna station (110) connected to the base station via the optical fiber transmission path (130), an opto-electric transducer (111) performs squared detection on the received optical signal. An amplifier (112) amplifies the resultant signal. The amplified signal is released from an antenna (113) into the air.
A spectrum (144) of this radio signal is shown in the figure. In other words, the signal is an image suppressed signal with a carrier frequency of f1-f2 (Hz) (e.g., a millimeter-wave frequency).
In the present method, the remote antenna station (110) does not need a radio-frequency filter for eliminating a lower sideband and the receiving terminal (120) does not require an oscillator, leading to a reduction in cost.
In the receiving terminal (120), the signal is received through an antenna (121) and is supplied to an amplifier and a band pass filter which are not shown. A detector (122) detects the signal using squared detection and supplies the detected signal to a signal demodulator (123). Multiplication two components of an unmodulated carrier (145) and a modulated radio signal component (146) of the radio signal (144) is obtained, so that an intermediate-frequency signal is reproduced. The intermediate-frequency signal is supplied to the signal demodulator (123) and is then demodulated, thus obtaining the information signal data.
To utilize the above-described radio optical fusion communication system in a multi-cell environment, it is preferred to reuse frequencies and switch a radio frequency channel in terms of interference between adjacent cells. However, there is no proposed method for switching a radio frequency channel.
The present invention is made in consideration of problems of the above-described related art and an object of the present invention is to provide a technique for easily switching a radio frequency channel at high speed.